1. Field of the Invention
The present invention is in the field of heart valve implantations. More particularly, the present invention is directed to artficial heart valves of the type which include a sewing ring to be permanently installed in the heart, and a tissue valve mounted to a stent removably attachable to the sewing ring.
2. Brief Description of the Prior Art
Two major types of heart valve implantations are known in the prior art. One of these, includes a metal or plastic artificial valve structure which regulates the direction of the blood flow in the anatomical structure wherein the heart valve is incorporated. By their very nature, these "mechanical type" heart valves have metal or plastic surfaces exposed to the blood flow, which remain thrombogenic even long time after their implantation by major surgery. Therefore, patients wearing such "mechanical type" heart valves can avoid potentially life threatening embolus formation only by constantly taking anti-thrombogenic medication, such as cumadine.
Another major type of artificial heart valve implant utilizes a tissue valve of animal (principally porcine) origin to regulate the direction of blood flow. Such porcine "tissue valves" include three cusps or leaves of a heart valve excised from pigs and preserved by treatment with glutaraldehyde. The animal tissue portion of the artificial "tissue valves" is, generally speaking, not thrombogenic. Therefore, at least some time after the surgical implantation of the artificial "tissue valves", the human patient would not necessarily require anti-thrombogenic medication (cumadine), unless, of course, some other portion of the artificial valve implantation includes thrombogenic metal or plastic surfaces exposed to the blood flow. Therefore, designers of the tissue valve type heart valve implants have, generally speaking, strived to minimize the plastic and metal surfaces of artificial heart valves which are exposed to blood flow.
Another important goal of artifical heart valve design is to minimize obstruction of blood flow in the anatomical structure wherein the artificial valve is implanted, i.e. to maximize the unobstructed flow passage area in the mitral, aortic or tricuspid annulus wherein the artificial valve is surgically mounted.
Still another goal is to render the "working" (flow direction regulating) valve structure replaceable without the need for surgically removing and destroying heart tissue in the annulus where the valve implant is mounted by sutures.
In efforts to attain the above-noted and related goals, the prior art has provided artificial heart valve implants which include a sewing ring and a valve structure removably mountable to the sewing ring. The sewing ring is, usually, a short section of a tube which is attached to a suitable biocompatible cloth or fabric. The sewing ring is surgically sewn (sutured) to the heart tissue.
The remainder of the artificial heart valve implant, whether it is the "mechanical" or "tissue valve" type, usually includes a support structure for the valve mechanism, with the support structure being securable by some type of locking mechanism to the sewing ring. The locking mechanism include threads in the sewing ring and complementary threads in the support structure. In addition to the foregoing, the prior art has also provided heart valve implants of the type wherein the sewing ring is integral (i.e. not removable during surgery), with the rest of the valve structure.
The valve support structure of the removable (and also non removable) tissue valve type heart valve implants usually includes a stent. The stents of the prior art usually comprise a bent wire structure made of a biocompatible, non-corrosive metal, such as stainless steel, or preferably a cobalt - nickel alloy known under the ELGILOY trade name. The stent typically includes a circular base and three stent posts (commissural posts) configured in such a manner that the three cusps of the porcine tissue heart valve can be attached to and are operatively supported by the commissural posts. In some prior art artificial valve implants, the stent is covered by a porous biocompatible fabric or cloth into which human tissue in-growth can occur after implantation, so as to reduce the risk of potentially dangerous blood clot formation.
Specific examples of prior art artifical heart valve implants are found in U.S. Pat. Nos. 3,744,062; 3,835,475; 3,997,923; 4,364,126 and 4,106,129. U.S. Pat. Nos. 4,211,325; 4,319,363 and 4,257,444 disclose subject matter which is of general background interest to the present invention.
The above-summarized prior art heart valves, including those disclosed in the above-cited patents, generally speaking, suffer from the following disadvantages. Plastic or metal surfaces of the valve implants remain exposed to the blood flow even in the "tissue valves" of the prior art, so that the patient wearing the valve implant must depend on anti-thrombogenic medication in order to avoid potentially lethal blood clot formation.
The configuration of the wire stent is often less than optimal for the mounting and prolonged functioning of the trileaflet (three-cusp) porcine valve. In addition, bent wire made of stainless steel, and particularly of cobalt - nickel alloy (ELGILOY), is not ideally suited for forming the stent from a structural integrity standpoint, even though ELGILOY is quite satisfactory from the standpoint of corrosion resistsance and biocompatibility. This is because, in order to provide adequate strength the wire must be fairly thick, and the thickness of the wire substantially reduces the flow-through area in the annulus wherein the valve implant is mounted. Moreover, in order to provide the necessary commissural posts in the stent, the wire of the stent must be subjected to relatively sharp bends which adversely affect its structural integrity. In this regard it is noted that for the making of certain stents for tissue valves, ELGILOY wire must be bent to a greater extent than what is considered acceptable by the manufacturers of the wire.
Still further, in the prior art heart valve prostheses which have a bent wire stent, an additional plastic support member must be mounted in the stent to operatively support the trileaflet porcine tissue valve. The need for this additional support member, of course, complicates the assembly of the heart valve prosthesis, and increases its cost.
Finally, the process of mounting the stent bearing the tissue valve, (or mechanical valve) to the sewing ring is difficult because of the conditions prevailing during open heart surgery, regardless of the structure of the valve implant. The prior art valve implants which had relatively complex mounting mechanisms have, generally speaking, rendered the task of the surgeon more difficult than desirable and exposed the patient to high risk.
In light of the foregoing, there is a genuine need in the prior art for improved artifical heart valve designs which overcome the above-noted disadvantages. The present invention provides such artificial heart valve designs.